Carbon scavenger fly ash pretreatment method

ABSTRACT

A method for pretreatment of a quantity of fly ash for use in a cementitious composition containing an air entrainment agent. An effective amount of an aromatic carboxylic acid or its salts is added to the fly ash to satiate unburned carbon in the ash. The treatment with the aromatic carboxylic acid (or hydroxy substituted aromatic carboxylic acid or its salts) does not significantly promote or retard the air entrainment functions of the air entrainment agent.

[0001] This application is a continuation of co-pending U.S. patentapplication Ser. No. 09/804,737, filed Mar. 13, 2001, which claimspriority to U.S. Provisional Patent Application No. 60/188,737, filedMar. 13, 2000.

BACKGROUND OF THE INVENTION

[0002] Coal fired power plants generate fly ash from the combustion ofpulverized coal, or combinations of pulverized coal and othercarbonaceous supplemental fuels such as petroleum coke, woodbark,charcoal, wood, residual fibers, etc. The fly ash is captured in thepower plant's emission control devices, such as by electrostaticprecipitators and baghouses. The principal composition of fly ashincludes an inert mineral fraction consisting primarily offerro-alumino-silicate glass and residual unburned carbon from coaland/or other supplemental fuels.

[0003] The inert mineral fraction of fly ash is a pozzolan, which makesfly ash an acceptable mineral admixture for use in Portland cementconcrete. The fly ash itself includes the devolatized mineral matterwhich has been trapped or loosely associated with the coal as well asincombustible components and elements of the coal and/or of thesupplemental fuels. In addition to such incombustible components, thefly ash contains carbon rich particles which have not been completelycombusted, usually due to the inefficiency of the boiler design orrelated conditions.

[0004] Specifications for the use of fly ash in Portland cement concreteare set out in ASTM #C618. This specification limits theloss-on-ignition (LOI) content of fly ash pozzolan to less than 6%. TheLOI value of fly ash in generally equal to the percent by weight of theunburned carbon content of the fly ash. This carbon content can varyfrom as little as about 0.5% up to 20% or more of the weight of thetotal fly ash product. However, for pozzolanic activity, a good qualityfly ash should contain less than 1% carbon but, in any case, no morethan 4% carbon.

[0005] A high carbon fly ash as a pozzolanic mixture has a detrimentalimpact upon the quality of concrete. The presence of carbon reduces airentrainment, which, in many locations, is the only real protection thatconcrete has against freeze-thaw/wet-dry conditions. Therefore, thelower the carbon content, the better the concrete mix is from an airentrainment perspective. The presence of carbon also increases waterrequirements, reduces pozzolanic reactivity, and degrades the appearanceof finished concrete surface. Thus, carbon negatively affects thestrength, durability, and aesthetic appearance of concrete. Therefore,the lower the carbon content in any fly ash, the better the fly ash as aconcrete admixture.

[0006] In many cases, air entrainment agents (AEA) are added to theconcrete mix in order to desirably increase the amount of small airbubbles that are formed during mixing. In these cases, carbon reducesthe ability of these agents to operate efficiently.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to a chemical treatment thatmay be applied to the high carbon content fly ash either prior to orconcurrent with its mixing with the other concrete components.

[0008] Specifically, aromatic carboxylic acid or hydroxy substitutedaromatic carboxylic acid is added to a quantity of fly ash to reduce airentraining chemical demand. Such aromatic carboxylic acids includebenzoic, phthalic, isophthalic, and terephthalic acids and their salts.Included in the hydroxy substituted aromatic carboxylic acids foundeffective are salicylic acid, m-hydroxybenzoic acid, andp-hydroxybenzoic acid and their salts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0009] Recent emphasis has been given to the removal of the carbon fromthe fly ash before utilization (i.e. mixing) of the ash component in theconcrete, such as Portland cement concrete (PCC). These chemicaltreatments or chemical carbon scavengers must somehow satisfy theaffinity of the carbon for the commonly used air-entraining agents. Atthe same time, the use of just any chemical is not acceptable since thechemical itself must not entrain air when the ash is used in the PCC,nor must the chemical inhibit the intended, desirable functions of theAEA.

[0010] Accordingly, “air neutral” agents must be used to improve the airentrainment properties of the carbon containing ash. That is, thesechemical carbon scavengers should not significantly promote airentrainment by themselves or retard air or bubble formation. However,these scavengers should be adsorbed onto the carbon, thus satisfying theaffinity or appetite that the carbon otherwise would have for the AEAs.

[0011] Pretreatment of the fly ash by the chemical carbon scavenger maybe performed simultaneously with other chemical treatments applied tothe ash, such as those that may be used to reduce the NH₃ content of theash.

[0012] At present, exemplary chemical carbon scavengers have been foundto include the aromatic carboxylic acids such as benzoic, phthalic,isophthalic, and terephthalic acids and their salts and the hydroxysubstituted aromatic carboxylic acids such as salicylic acid,m-hydroxybenzoic acid and p-hydroxybenzoic acid and their salts.

[0013] Based upon available preliminary test data, salicylic acid andits salts may be preferred, with lithium salicylate perhaps the mostpreferred at present. It has been noted that treatment of 100 g of ashwith 0.07 g lithium salicylate has achieved at least 30% reduction ofair entraining chemical demand. Further, this particular carbonscavenger did not exhibit either air entrainment or suppression. It isanticipated that treatment dosages of these scavengers will range from0.01 g to 100 g chemical carbon scavenger based on 100 g of fly ash.

[0014] Other additional candidate chemical carbon scavengers includefamilies of dyes and multi-ring aromatics as well as some alkyl-arylcompounds.

[0015] Although the invention has been described with reference to aspecific embodiment, this description is not meant to be construed in alimiting sense. On the contrary, various modifications of the disclosedembodiments will become apparent to those skilled in the art uponreference to the description of the invention. It is thereforecontemplated that the appended claims will cover such modifications,alternatives, and equivalents that fall within the true spirit and scopeof the invention.

1. A method for pretreating fly ash for use in cementitious compositionscontaining an air entrainment agent comprising the steps of: applying aneffective amount of benzoic acid to a quantity of fly ash to satiateunburned carbon in said ash without effecting the air entrainmentfunctions of said air entrainment agent.
 2. A method for pretreating flyash for use in cementitious compositions containing an air entrainmentagent comprising the steps of: applying an effective amount of salicylicacid to a quantity of fly ash to satiate unburned carbon in said ashwithout effecting the air entrainment functions of said air entrainmentagent.
 3. The method of claim 1 wherein said amount of benzoic acid isin the range of about 0.01 g to about 100 g for each 100 g of said flyash.
 4. The method of claim I wherein said amount of salicylic acid isin the range of about 0.01 g to about 100 g for each 100 g of said flyash.
 5. A method for pretreating fly ash for use in cementitiouscompositions containing an air entrainment agent comprising the stepsof: applying an effective amount of lithium salicylate to a quantity offly ash to satiate unburned carbon in said ash without effecting the airentrainment functions of said air entrainment agent, said effectiveamount of lithium salicylate is about 0.07 g for each 100 g of said flyash.